Appendix A: Technology Descriptions
Conventional Pulverised Coal with FGD
In a conventional plant pulverised coal is burnt in a boiler to produce steam, which is fed into a steam turbine coupled to an electrical generator. Emission controls such as electrostatic precipitators or bag houses, and flue gas desulphurisation (FGD) limit pollutants to permitted levels. A typical example of this type of plant is Huntly power station except that it does not have FGD because of low sulphur levels in NZ coals.
Expected changes with time will be increased steam temperatures and pressures leading to higher efficiencies coupled with more effective environmental controls.
Fluidised Bed Coal Combustion
Fluidised beds suspend solid fuels on upward-blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids, much like a bubbling fluid. The mixing action of the fluidised bed brings the flue gases into contact with a sulphur-absorbing chemical, such as limestone or dolomite. More than 95 percent of the sulphur pollutants in coal can be captured inside the boiler by the sorbent.
Fluidised bed boilers can burn almost any combustible material, from coal to municipal waste, and are capable of meeting sulphur dioxide and nitrogen oxide emission standards without the need for expensive add-on controls.
Integrated Coal Gasification Combined Cycle (ICGCC)
Rather than burning coal directly, coal gasification reacts coal with steam and controlled amounts of air or oxygen under high temperatures and pressures to produce a gaseous mixture, typically hydrogen and carbon monoxide. These hot, coal gases exiting the gasifier are used to power a gas turbine (in the same manner as natural gas). Hot exhaust from the gas turbine is then fed to a heat recovery steam generator (HRSG). The steam from the HRSG is then fed to a conventional steam turbine, producing a second source of power (just as in a combined cycle plant).
Pollutant-forming impurities and greenhouse gases can be separated from the gaseous stream. Unreacted solids can be collected and marketed.
Gasification is used today in refineries and chemical plants, but the technology is still in the demonstration phase for electric power generation.
Combustion Turbine (Gas Turbine)
Combustion of the fuel produces a high-temperature, high-pressure gas working fluid. When this is exhausted through a gas turbine this causes the shaft to rotate by expanding the gas through a series of specially designed blades. The rotating shaft drives an electric generator and a compressor for the inlet air used by the gas turbine. Many turbines also use a heat exchanger called a recuperator to add turbine exhaust heat into the combustor's air/fuel mixture.
Gas turbines are compact, lightweight, easy to operate, and come in sizes ranging from several hundred kilowatts to hundreds of megawatts.
Examples of this type of plant are the old gas turbines at Whirinaki and Otahuhu power stations. Current gas turbines are more efficient.
Advanced Combustion Turbine
This type of combustion turbine can operate at higher temperatures through the use of more exotic materials and other enhancements to achieve higher efficiencies.
Gas Combined Cycle
Fuel, generally natural gas but can be other gaseous or liquid fuels, is burned in a gas turbine coupled to an electrical generator. Exhaust heat from the gas turbine is then passed into a heat recovery steam generator (HRSG), which can be fired or unfired. The steam is then fed to a conventional steam turbine to provide a second source of power. Otahuhu B Power Station and the most recent Stratford Power Station are examples of this type of plant but at the time of ordering they would have been considered to be more in the advanced class of combined cycle plant.
Advanced Gas Combined Cycle
This is a type of combined cycle plant utilising higher temperatures through the use of more exotic materials and other enhancements to achieve higher efficiencies.
Distributed Generation
Distributed Generation (DG) strategically applies relatively small generating units (typically less than 30 MWe) at or near consumer sites to meet customer needs, to support economic operation of the existing power distribution grid, or both. DG technologies cover a range of renewable and thermal plant options.
Thermal technologies such as gas turbines and reciprocating engines are already making a contribution. Microturbines and fuel cells are beginning to enter the market, but will require additional research and development to realize widespread adoption. Also, fuel cell/turbine hybrid systems and 21st century fuel cells, currently in the embryonic stage, offer even greater potential.
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